The molecular genetics of curd morphology and the domestication of cauliflower (Brassica oleracea L. var. botrytis L.)
The characteristic curd phenotype of the Brassica cauliflower consists of proliferating, arrested inflorescence and floral meristems. Recent analysis of the similar phenotype in the ap1-1/cal-1 mutant of the related crucifer Arabidopsis thaliana has led to speculation that the orthologous genes from Brassica oleracea L. may be responsible for this characteristic trait. Application of molecular genetic analysis to this hypothesis allows the presentation of a genetic model based on specific, mapped loci of BoCAL and BoAP 1. This model accounts for differences in the stage of arrest between the heading phenotypes of cauliflower (B. oleracea var. botrytis L.) and Calabrese broccoli (B. oleracea var. italica Plenck), and is also predictive in accounting for intermediate stages of arrest similar to those observed in Sicilian Purple types. Further molecular genetic analysis characterised three independent loci of the floral meristem identity gene BoAP 1. Integration of this data into the genetic model proposed for curd development, suggests a combination of point mutations and expression thresholds of several copies of the key meristem identity genes BoCAL and BoAP 1 respectively may account for the development of curd tissue in the Brassica cauliflower. The association of alleles of the BoCAL-a gene with the curding phenotypes of B. oleracea was also demonstrated through a survey of over 200 crop accessions. This reveals strong correlations between specific BoCAL-a alleles and discrete inflorescence morphologies, and allows the presentation of a possible scenario for the domestication of cauliflowers. Molecular genetic analysis of BoCAL-a utilising monosomic addition lines has also demonstrated the potential for integration of the genetic and cytogenetic maps of B. oleracea. Such analysis may have significant utility for physical characterisation of replicated loci in B. oleracea, prior to the development of a strong physical map. Further examination of inflorescence morphologies amongst the heading brassicas revealed a shared trait, termed Fused Inflorescence. Preliminary investigation suggests this trait may be under the control of multiple loci, providing a possible indication of the delineation between heading and sprouting B. oleracea crops.